Switching power converters have many applications. For example, mobile information technology devices, such as mobile telephones, tablet computers, and notebook computers, typically include a number of DC-to-DC switching power converters, such as buck and/or boost DC-to-DC converters, to regulate power and/or to perform voltage level transformation. As another example, stationary computing devices, such as servers and desktop computers, often include an AC-to-DC switching power converter, such as an isolated forward converter, half-bridge converter, or flyback converter, to power the device from an AC power source. Stationary computing devices also typically include one or more DC-to-DC converters to perform local power conversion within the device, such as to transform bulk DC power to a form suitable for powering electronic circuits.
FIG. 1 illustrates a prior-art buck DC-to-DC converter 100, which represents conventional buck converters used in many applications. Converter 100 includes an inductor 102, a switching circuit 104, a controller 106, and a filter capacitor 108. Controller 106 causes transistors 110, 112 of switching circuit 104 to switch between their conductive and non-conductive states to generate a switching waveform form, which is filtered by inductor 102 and capacitor 108, and thereby transfer power from an input power port 114 to an output power port 116. Controller 106 is often capable of controlling switching circuit 104 to regulate voltage Vo across output power port 116 and/or current Io delivered to a load (not shown) electrically coupled to output power port 116.
FIG. 2 shows a curve 200 plotting estimated efficiency versus output current Io for DC-to-DC converter 100. As can be seen, converter 100 realizes maximum efficiency at output current magnitude I1. Efficiency drops off somewhat at heavy loads beyond I1. However, efficiency drops off significantly at light loads below I1. Thus, converter 100 is relatively inefficient at light loads.
Efficient operation at light load is important since electronic devices are typically designed to operate in low power states when full functionality is not required. For example, computer microprocessors typically operate in a low power mode, such as a “sleep mode,” when the processor is performing little activity. As another example, mobile telephones often operate in a low power standby mode when idle. Many electronic devices will spend much time in a low power state, and light load efficiency is therefore an important factor in device power consumption.
One known technique for improving light load efficiency is to reduce the size of switching transistors, such as transistors 110, 112 in switching circuit 104 (FIG. 1). Reducing transistor size generally reduces transistor parasitic capacitance and transistor driving requirements, thereby reducing switching related losses. Such technique, however, reduces heavy load efficiency, which is unacceptable in many applications where the converter must power heavy, as well as light, loads. It is also known that light load efficiency can sometimes be increased by increasing the value of energy storage inductance, such as by increasing the inductance value of inductor 102 (FIG. 1). However, increasing inductance impairs the converter's transient response, and may also reduce the converter's heavy load efficiency. Accordingly, increasing inductance value is not an acceptable option in many applications.